PAIN 467 



sensations of pain are aroused by a sort of summation taking place in the 

 cells of the gray matter, and there is any number of observations at hand 

 which show that tactile stimuli, of themselves wholly painless, produce severe 

 pain if repeated frequently enough. Likewise the irradiation of pain, as 

 well as the occurrence in pathological processes of many accessory sensations 

 of a painful character, appear to speak for the participation of the gray 

 matter. 



While these and other observations can be explained on the ground that 

 the sensory cutaneous nerves already discussed mediate the sensations of pain, 

 they do not, however, constitute positive proof of that proposition. Let us 

 see what we may learn from investigation of the different sense points of 

 the skin. 



There prevails among authors who have busied themselves with this ques- 

 tion a most gratifying agreement on one point, namely, that neither stimula- 

 tion of the temperature points by their appropriate stimuli nor mechanically 

 (by a needle thrust) produces any pain ( Goldscheider et al.). Likewise when 

 a heat spot is tested with very warm water, it gives a burning hot sensation 

 but no pain. The heat pain might be regarded therefore as a separate sensa- 

 tion of pain merely colored by the excitation of the heat nerves, unless we 

 suppose that the analgesia of the heat spots is due to the fact that the surface 

 stimulated is too small ; for it is a well-known fact that the size of the surface 

 stimulated is a very important factor in the production of pain. 



Blix demonstrated that on many parts of the body a needle can be thrust 

 deep into the skin without producing any pain. The nerves which mediate 

 pain therefore do not occur everywhere in the skin. Neither Blix nor Gold- 

 scheider however felt impelled to assume the existence of special nerves of 

 pain with their own end organs, but conceived that sensations of pain have 

 their origin in excitation of pressure nerves, v. Frey, on the other hand, 

 entered the lists for special pain nerves and adduced the following weighty 

 reasons, among others, for their existence. 



(1) By observing certain precautions, mechanical stimulation of the skin 

 with a bristle produces a pure sensation of pain without any preliminary or 

 accompanying sensation of pressure. (It will be readily understood that the 

 pain spots cannot be stimulated singly by mechanical means when they lie in 

 the immediate neighborhood of pressure points.) 



(2) If a bristle be placed over a pressure point, the sensation appears imme- 

 diately, but at once fades away again and usually becomes unnoticeable after a 

 short time. Over the pain point the effect appears later, gradually increases in 

 strength and decreases again after reaching a maximum. If the sensation is 

 still present after the stimulus has ceased, it disappears slowly. Intimately con- 

 nected with this behavior is the fact that rapidly repeated electrical or mechani- 

 cal stimuli (from five per second upward) applied to the pain point fuse as a 

 rule into a continuous sensation, whereas through the pressure point we can 

 distinguish very well 130 shocks per second (page 462). 



(3) When the head of a pin is pressed for a moment into the skin, there 

 follows very often after the sensation of pressure and separated from it by a 

 short interval, a second sensation which is painful. Only pain points in the 

 neighborhood of pressure points exhibit this phenomenon. On isolated pressure 

 points the painful after-effect is wanting, and on isolated pain points the sen- 



